1. Field of the Invention
The invention relates generally to a digital watch having an infrared plethysmograph and, in prticular, to a remote pulse sensor cable for use with a pulse sensing digital watch.
2. Description of the Prior Art
Pulse sensing digital watches are relatively new to the field of heart rate monitoring but, nonetheless, their principles of operation are well known and understood. One of the first pulse sensing watches on the market utilizes an infrared pulse sensor mounted on a watch face and connected to special electronics within the watch. The wearer activates the plethysmograph electronics merely by pushing a button switch on the edge of the watch case, then placing his fingertip over the infrared sensor, and reading his pulse rate directly from the digital display.
Pulse rate sensing is dependent upon certain physiological changes which occur during each cardiac cycle, i.e., the diastole and the systole. In the diastole phase the cavities of the heart expand and fill with blood. The diastolic pressure is the lowest arterial blood pressure of a cardiac cycle occurring during the diastole of the heart. In the systole phase, the heart contracts, forcing the blood onward thus keeping the circulation up. The systolic pressure is the highest arterial pressure of a cardiac cycle. The fresh blood supply from the heart is conducted by arteries, and thence by capillaries. Veins return the blood supply to the heart. Blood in the arteries and capillaries is under pressure and flows in waves due to the beats of the heart. In response to the systole of the heart, the pressure in the arterial/capillary system increases to its maximum value and the system fills with the blood being pumped out of the heart. During diastole, the heart fills with blood from the veins as the pressure drops in the arterial/capillary system and the amount of blood in this system decreases.
One's finger tips contain a great number of these tiny capillaries which fill with a fresh blood supply during the systolic phase and empty during the diastolic phase. Heart-rate measurement relies on the slight increase in infrared light absorption by the blood in the capillaries of the fingertip during the systolic pressure wave.
An infrared plethysmograph within a digital watch may include a light emitting photodiode which emits either a continuous or a pulsed infrared signal which is directed at the capillaries in the fingertip. The IR reflected from the capillaries is detected by an infrared detector such as a photodiode or phototransistor. The IR detector is coupled to a microcomputer within the watch case. As explained above, the capillaries are more reflective of IR energy during the diastole than the systole. The microcomputer measures the differences in the signals reflected by the capillaries, counts the intervals between them, amplifies the data, averages the calculated heartbeat and displays it periodically after a predetermined number of heartbeats.
The reasons for wearing such a pulse sensing watch may be as varied as the number of individuals wearing them. But generally, persons are interested in knowing their pulse while at rest, to indicate their degree of relaxation, or during some activity, to determine, at least indirectly, the stress they place on their hearts. Sampling one's own pulse rate at rest is a simple procedure. The wearer merely turns the sensing electronics on, places his finger over the infrared sensor gently, and reads his pulse directly from the display. It is necessary to apply a constant and light finger pressure against the infrared sensor, otherwise false readings may occur. If too much pressure is applied, circulation through those capillaries may be severely curtailed and low pulse readings would result. If insufficient finger pressure is applied, the wearer's finger may move relative to the sensor thereby giving false readings.
In order for one to have a more accurate reading of one's pulse during an activity, the pulse should be read during that activity. Although a high heart-beat rate during a strenuous activity is not immediately reduced upon cessation of that activity, the heart does tend to slow down rapidly when the activity is terminated. Therefore, it is most desirable to sample one's pulse during the most strenuous phases for an accurate determination of the stress one places on one's heart.
Monitoring one's pulse in the course of some activities may be difficult because of the pressure requirements explained above. For example, a runner while running places a finger of his right hand on the wristwatch sensor worn on the left arm and hopes that he is applying the proper pressure. This is a rather clumsy and awkward attitude. More than likely the readings obtained would be inaccurate due to the pressure requirements. In order to obtain precise readings, that person would have to stop and take his pulse. However, when he stops, his heart has started its slowing process and the readings he obtains are not representative of his previously higher heartbeat.